Falls in the elderly are common and are hazardous. Almost one-third of community-dwelling persons 75 or more years old fall each year and more than five percent of those who fall experience fractures. The tendencies of old adults to fall is associated with increases in their postural sway. Both fall and postural sway responses, regardless of underlying psychological, neurological or musculoskeletal disorders, can be quantified in biomechanical terms. Continuation is proposed of research to examine the response biomechanics of the musculoskeletal system in the arrest of impending falls and the maintenance of standing postural balance in the presence of perturbations. Various disturbances of standing posture will be imposed. Sway reactions and stepping reactions will result. Body segment kinematics, myoelectric activities and support surface reactions will be measured. Joint torques will be computed through biomechanical model analyses. The overall objectives of Project 4 are to quantify differences in disturbance response biomechanics among healthy young and old adults and mobility-impaired old adults both with and without Parkinson's Disease. The proposed research will seeks specific biomechanical sources of those differences and relate them to the biomechanical and neuropsychological outcome measures obtained by other Program units. We will test a number of hypotheses concerning these differences. Understanding of how responses to postural perturbations are affected by natural aging, by neuropsychological status and by impairment will enable earlier and more precise detection of declines in mobility-related task performance capabilities. It will show what aspects of impairment are truly critical to postural maintenance in the presence of disturbances and thus lead to improved assessment and intervention.